As a technique to increase a density of an inkjet head using a piezoelectric element, a technique is known and embodied which applies a micro electromechanical system (below abbreviated as “MEMS”). In other words, a semiconductor device manufacturing technique may be applied and an actuator and a liquid flow path may be minutely formed to increase a nozzle density, making it possible to realize a reduced size and an increased density of the head.
In the inkjet head adopting such an MEMS technique, using photolithography, an electrode and a piezoelectric material which are formed with a thin-film forming technique may be patterned on a vibrating plate which is formed with a thin-film technique, and a piezoelectric element may be formed to make an actuator. In this case, the piezoelectric element is patterned using a semiconductor process, so that a thickness of the piezoelectric material is limited to a few micrometers at the most. Moreover, for etching or forming of a wiring electrode or an insulator film needed for a device, or an electrode which forms the piezoelectric element, a process using plasma, i.e., a plasma CVD, dry etching, etc., is commonly used.
When the piezoelectric element (more specifically, when a below-described PZT, referring to “a solid solution of titanic acid (PbTiO3) and lead zirconate (PbTiO3)” or “lead zirconate titanate”, is used as the material) is exposed to the above-described plasma process, the piezoelectric material is reduced by a reduction action of hydrogen, etc., which are generated during the process, so that the characteristics tend to deteriorate remarkably. Moreover, besides the above-described plasma process, it is commonly known that the characteristics of the piezoelectric material deteriorate due to moisture within the atmosphere.
As countermeasures for the above-described problems, techniques are being proposed which cover an end or the whole faces of the piezoelectric element with a protective layer (see Patent documents 1 and 2, for example).
Patent document 1 discloses a technique such that a piezoelectric element may be covered with an inorganic amorphous material to prevent penetration of moisture into a piezoelectric material, enhancing reliability of the piezoelectric material. Moreover, it discloses that, when extending a lead electrode formed on the inorganic amorphous material via a contact hole from an upper electrode and connecting to a drive circuit, an insulating film which is different from the above-described inorganic amorphous material is covered on the lead electrode, making it possible to use an electrode material such as Al (Aluminum), which tends to corrode. In this way, an inexpensive wiring material may be used. When the lead electrode is drawn around the inorganic amorphous material, a layout may be adopted such that it overlaps a lower electrode (common electrode).
On the other hand, Patent document 2 discloses a technique such that, in an insulating film formed on a piezoelectric element, an inorganic material layer and an organic material layer are stacked. In other words, an end of a piezoelectric material into which moisture tends to penetrate is covered with the inorganic material, while at the same time an opening is provided on an upper electrode, making it possible to suppress, to a minimum level, a drop in an amount of vibratory displacement due to a rigid inorganic material as well as to prevent moisture permeation at the same time. Moreover, the whole face of the piezoelectric element is covered with a soft organic material, making it possible to ensure reliability of a device.
Patent Document
Patent document 1: JP2010-042683A
Patent document 2: JP4371209B
However, with the technique disclosed in Patent document 1, as the whole pattern area face including the piezoelectric element is covered with the inorganic amorphous material, making a film thick remarkably obstructs a displacement of the piezoelectric element, causing the ejection characteristics to deteriorate considerably. On the other hand, making a thin film of the inorganic amorphous material in order to achieve a large amount of displacement of the piezoelectric element causes an inability to ensure withstand pressure between the lead electrode and the lower electrode. Therefore, there is a problem that, as it is necessary to provide an electrode layout such that an overlap of the lead electrode and the lower electrode does not occur, making a head small and highly dense becomes difficult, and, at the same time, a constraint occurs on a height of a junction with a protective substrate, making an enhancement of the junction quality difficult. In a device manufactured in a semiconductor process, making an element highly dense is an important problem since it affects manufacturing costs. In other words, this is because the number of chips cut out from a single wafer greatly affects the costs.
Also in the technique disclosed in Patent document 2, two layers of insulating films are formed on a piezoelectric material, causing a tendency for vibration hindrance to occur. Moreover, in order to ensure withstand pressure with the insulating film of an organic material, it is necessary to thicken the film relative to a general inorganic material and, at the same time, adhesion with an electrode material is poor, so that it is difficult to form a lead electrode on the organic material. Therefore, the lead electrode is formed between the organic material (insulating film) and the inorganic material (insulating film); however, as described above, with such a configuration, the lower electrode and the lead electrode cannot be overlapped as described above (or a film thickness of the inorganic material is needed such that an amount of displacement of the piezoelectric element drops remarkably), so that making a head highly dense becomes difficult.